I would have done something like this:
{code}
ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("ClsSync");
CtMethod m = cc.getDeclaredMethod("biat");
int oldMod = m.getModifiers();
int mod = Modifier.clear(oldMod, Modifier.SYNCHRONIZED);
m.setModifiers(mod);
Class newClass = cc.toClass(); // or cc.writeFile() if you wish to keep the changes
{/code}